Moderate Current Densities Research Articles

The effects of temperature dependent recombination rates on performance of InGaN/GaN blue superluminescent light emitting diodes

The effects of temperature dependent radiative and nonradiative recombination (Shockley–Read–Hall, spontaneous radiative, and Auger coefficients) on the spectral and power characteristics of a blue multiple quantum well (MQW) superluminescent light emitting diode (SLD or SLED) have been studied. The study is based on the rate equations model, where three rate equations corresponding to MQW active region, separate confinement heterostructure (SCH) layer, and spectral density of optical power are solved self-consistently with no k-selection energy dependent gain and quasi-Fermi level functions at steady state. We have taken into account the temperature effects on Shockley–Read–Hall (SRH), spontaneous radiative, and Auger recombination in the rate equations and have investigated the effects of temperature rising from 300K to 375K at a fixed current density. We examine this procedure for a moderate current density and interpret the spectral radiation power and light output power diagrams. The investigation reveals that the main loss due to temperature is related to Auger coefficient.

In-Plane Distribution Analysis in the Land-Channel Direction of a Proton Exchange Membrane Fuel Cell (PEMFC)

A novel design was implemented to segment a proton exchange membrane fuel cell (PEMFC) in the land-channel direction to measure the current density distribution at sub-millimeter resolution of 340 μm. This design was validated by performing an ex-situ electrical resistance test, in-situ hydrogen pumping test, and by comparing performances between non-segmented and segmented cells at two different conditions. Further, this design was utilized to measure the current density distribution in wet conditions, in extremely dry condition, and in moderate condition. In wet conditions current densities are found to be higher in the channel areas than in the land areas. To differentiate between the effects of transport distances from channel to the catalyst layer and non-uniform water saturation distribution in gas diffusion medium, oxygen concentration ratios were estimated in wet conditions. Predicted oxygen ratio distribution showed that the effect of water accumulation under the land in gas diffusion medium is more significant than the effect of transport distance which can be well supported by in-situ microscopy study published in literature. However, in dry condition, current density was higher in the land areas than in the channel while in moderate condition current density was found to be uniform.

Green (In,Ga,Al)P-GaP light-emitting diodes grown on high-index GaAs surfaces

We report on green (550–560 nm) electroluminescence (EL) from (Al0.5Ga0.5)0.5In0.5P-(Al0.8Ga0.2)0.5In0.5P double p-i-n heterostructures with monolayer-scale GaP insertions in the cladding layers and light-emitting diodes based thereupon. The structures are grown side-by-side on high-index and (100) GaAs substrates by molecular beam epitaxy. At moderate current densities (∼500 A/cm2), the EL intensity of the structures is comparable for all substrate orientations. Opposite to the (100)-grown strictures, the EL spectra of (211) and (311)-grown devices are shifted towards shorter wavelengths (∼550 nm at room temperature). At high current densities (>1 kA/cm2), a much higher EL intensity is achieved for the devices grown on high-index substrates. The integrated intensity of (311)-grown structures gradually saturates at current densities above 4 kA/cm2, whereas no saturation is revealed for (211)-grown structures up to the current densities above 14 kA/cm2. We attribute the effect to the surface orientation-dependent engineering of the GaP band structure, which prevents the escape of the nonequilibrium electrons into the indirect conduction band minima of the p-doped (Al0.8Ga0.2)0.5In0.5P cladding layers.

Analysis of non-isothermal effects on polymer electrolyte fuel cell electrode assemblies

A non-isothermal, single phase membrane electrode assembly (MEA) mathematical model accounting for most applicable heat sources, viz., reversible, irreversible, ohmic heating, phase change, heat of sorption/desorption, is presented. The mathematical model fully couples a thermal transport equation with an MEA model and allows the study of non-isothermal effects, such as thermal osmosis through the membrane, local relative humidity variations in the catalyst layers and water sorption into the membrane. A detailed breakdown of various heat sources in the MEA at different current densities is provided and the impact of various thermal effects previously neglected in the literature such as thermal-osmosis, reversible heat distribution, and heat of sorption are studied. Results show that sorption heat cannot be neglected as it contributes up to 10% of the total heat under normal operating conditions. Reversible heat distribution can significantly affect the temperature distribution shifting the hottest location of the cell from anode and cathode. Analyzing the water transport across the membrane, results show that thermal-osmosis contributes up to 25% of the water flux inside the membrane at moderate and high current densities.

Impact of Load Cycling at High Current Densities on the Degradation Behavior of Membrane-Electrode-Assemblies

For the establishment in the field of renewable energy and energy converters, High Temperature Polymer Electrolyte Membrane fuel cells (HT-PEMFC) need, next to an increase of performance, an improvement in their lifetime expectancy. The amendment of both, durability and performance, requires a better understanding of degradation effects and – processes within the fuel cell. The term degradation can be defined as a decrease of performance as a function of operation time. The degeneration of fuel cells can be caused by thermal, chemical and mechanical degradation. The debasement due to mechanical stress caused by severe operational conditions lies in the focus of this work. Accelerated stress tests (AST) at high current densities will be carried out with membrane electrode assemblies (MEA) from two different providers. AST are a good instrument to determine the durability of a MEA and its components in a shorter time span. A standard AST is an operation procedure with a load cycling between moderate current densities (0.3 – 0.6 A/cm²) and open circuit voltage (OCV). In our case, we will concentrate on load cycling treatment with higher current densities between 0.6 and 1.0 A/cm². The operation mode is presented in figure 1. The highest current (1.0 A/cm²) density will be kept for 16 min and the holding time of the lower value (0.6 A/cm²) will be 4 min. One cycle lasts 6 hours and ends with a 10 min OCV phase. The development of the OCV is a good indicator of the state of the membrane (1). After 17 hours of load cycling, the MEA will be characterized with electrochemical measurements like polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) (2). After 7 hours of characterization, the load cycling takes again place in the operation procedure. This method will be repeated day by day for one week.Next to the electrochemical characterization, selected MEAs will be also analyzed ex-situ by imaging procedures like Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and micro-computed tomography (µ-CT). The combination of electrochemical and imaging investigations allows a more detailed and holistic research of degradation effects and defects within the MEA.With support of the µ-CT, structural changes within the MEA due to the high current densities can be observed. Expected defects like for example pin holes and fiber intrusion can be visualized. These errors (and their influences) on the fuel cell performance can be analyzed and confirmed by LSV measurements (3).The electron microcopy facilitates the examination of microstructure changes, for example cracking of the catalyst layer or delamination (4). TEM measurements permit the ante and post mortem investigations of the catalyst-particle-formation (5) and the influence of the advanced AST on this development. Figure 1 : Overview of load cycling testing procedure at high current densities

Indirect Electrochemical Reduction of Indigo on Carbon Felt: Process Optimization and Reaction Mechanism

A carbon felt was used as the cathode material for the indirect electrochemical reduction of indigo in alkaline aqueous solution employing iron-triethanolamine (Fe-TEA) complex as a redox mediator. Surface structure and specific surface area of the carbon felt electrode and its electrochemical performance toward the reduction of indigo were investigated by SEM, BET, and cyclic voltammetry, respectively. Moreover, an optimization of different operating conditions for the reduction of indigo was performed with an H-cell. In particular, the dependence of current efficiency (CE) on electrolysis time was chosen to analyze the indirect electrochemical reduction mechanism of indigo. The results showed that the carbon felt, having a high specific surface area of approximately 7289 cm2·g–1, is an excellent electrode material for the indirect electrochemical reduction of indigo and that a moderate current density, temperature, and concentration of indigo has the most beneficial effects on the reduction process.

A real-time simulating non-isothermal mathematical model for the passive feed direct methanol fuel cell

ABSTRACTIn order to understand the complex transport phenomena in a passive direct methanol fuel cell (DMFC), a theoretical model is essential. The analytical model provides a computationally efficient framework with a clear physical meaning. For this, a non-isothermal, analytical model for the passive DMFC has been developed in this study. The model considers the coupled heat and mass transport along with electrochemical reactions. The model is successfully validated with the experimental data. The model accurately describes the various species transport phenomena including methanol crossover and water crossover, heat transport phenomena, and efficiencies related to the passive DMFC. It suggests that the maximum real efficiency can be achieved by running the cell at low methanol feed concentration and moderate current density. The model also accurately predicts the effect of various operating and geometrical parameters on the cell performance such as methanol feed concentration, surrounding temperature, and polymer electrolyte membrane thickness. The model predictions are in accordance with the findings of the other researchers. The model is rapidly implementable and can be used in real-time simulation and control of the passive DMFC. This comprehensive model can be used for diagnostic purpose as well.

The Effect of Current Density on the NiCu Coating Synthesized by Electroplating

In this paper, the NiCu alloy coatings were electroplated on the 304 stainless steel coupon. The XRD was used to identify the phase structure of the as-prepared products. The SEM and EDS were used to observe the surface morphology of the as-prepared coatings and the atomic composition of the as-prepared coatings, respectively. The influences of the current density on the as-plated NiCu coatings are systematically researched. It was found that the moderate current density is beneficial to the compactness of the coatings. The content of the copper decreased and that of the nickel increased with the increasing of the current density. When the current density is smaller than 40 mA/cm2, the contents of nickel and copper vary apparently. When the current density is larger than 40 mA/cm2, the contents of nickel and copper vary slightly. That means the copper is more easily deposited at low current density and the nickel is more easily deposited at high current density.

Mechanistic modeling of polysulfide shuttle and capacity loss in lithium–sulfur batteries

Lithium–sulfur (Li/S) cells are promising candidates for a next generation of safe and cost-effective high energy density batteries for mobile and stationary applications. At present, most Li/S cells still suffer from relatively poor cyclability, capacity loss under moderate current densities and self-discharge. Furthermore, the underlying chemical mechanisms of the general discharge/charge behavior as well as Li/S-specific phenomena like the polysulfide shuttle are not yet fully understood. Here we present a thermodynamically consistent, fully reversible continuum model of a Li/S cell with simplified four-step electrochemistry, including a simple description of the polysulfide shuttle effect. The model is parameterized using experimental discharge curves obtained from literature and reproduces behavior at various current densities with fairly high accuracy. While being instructively simple, the presented model can still reproduce distinct macroscopic Li/S-cell features caused by the shuttle effect, e.g., seemingly infinite charging at low charge current densities, and suboptimal coulombic efficiency. The irreversible transport of active material from the cathode to the anode results in a voltage drop and capacity loss during cycling, which can also be observed experimentally.

Influence of the anodic etching current density on the morphology of the porous SiC layer

In this report, we fabricated a porous layer in amorphous SiC thin films by using constant-current anodic etching in an electrolyte of aqueous diluted hydrofluoric acid. The morphology of the porous amorphous SiC layer changed as the anodic current density changed: At low current density, the porous layer had a low pore density and consisted of small pores that branched downward. At moderate current density, the pore size and depth increased, and the pores grew perpendicular to the surface, creating a columnar pore structure. At high current density, the porous structure remained perpendicular, the pore size increased, and the pore depth decreased. We explained the changes in pore size and depth at high current density by the growth of a silicon oxide layer during etching at the tips of the pores.

Performance evaluation of a direct methanol fuel cell (DMFC) with novel anode flow field operating with neat methanol

This paper reports the structural effect of a novel anode flow field on the performance of a direct methanol fuel cell (DMFC) operating with neat methanol. The anode flow fields with various vaporization channel configurations, open ratios, gap aspect ratios and vaporization channel lengths are designed, fabricated and tested on DMFCs. Results show that the DMFC with an anode flow field with central vapor inlet yields better performance than that with vapor inlet in the corner. Both the open ratio and aspect ratio of the anode flow field have significant influences on cell performance. Larger open ratio and larger aspect ratio lead to more uniform reactant distribution in the in-plane direction, but can result in a lower methanol concentration in the anode catalyst layer (ACL), which makes the DMFC suffers larger polarization loss at moderate and high current densities. In addition, it is found that a longer vaporization channel facilitates the cell to work at lower temperature, which is promising to operate a neat-methanol-fed DMFC by self-heating.

Benzothiadiazole-based polymer for single and double junction solar cells with high open circuit voltage.

Single and double junction solar cells with high open circuit voltage were fabricated using poly{thiophene-2,5-diyl-alt-[5,6-bis(dodecyloxy)benzo[c][1,2,5]thiadiazole]-4,7-diyl} (PBT-T1) blended with fullerene derivatives in different weight ratios. The role of fullerene loading on structural and morphological changes was investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD). The XRD and AFM measurements showed that a higher fullerene mixing ratio led to breaking of inter-chain packing and hence resulted in smaller disordered polymer domains. When the PBT-T1:PC60BM weight ratio was 1 : 1, the polymer retained its structural order; however, large aggregated domains formed, leading to poor device performance due to low fill factor and short circuit current density. When the ratio was increased to 1 : 2 and then 1 : 3, smaller amorphous domains were observed, which improved photovoltaic performance. The 1 : 2 blending ratio was optimal due to adequate charge transport pathways giving rise to moderate short circuit current density and fill factor. Adding 1,8-diiodooctane (DIO) additive into the 1 : 2 blend films further improved both the short circuit current density and fill factor, leading to an increased efficiency to 4.5% with PC60BM and 5.65% with PC70BM. These single junction solar cells exhibited a high open circuit voltage at ∼ 0.9 V. Photo-charge extraction by linearly increasing voltage (Photo-CELIV) measurements showed the highest charge carrier mobility in the 1 : 2 film among the three ratios, which was further enhanced by introducing the DIO. The Photo-CELIV measurements with varying delay times showed significantly higher extracted charge carrier density for cells processed with DIO. Tandem devices using P3HT:IC60BA as bottom cell and PBT-T1:PC60BM as top cell exhibited a high open circuit voltage of 1.62 V with 5.2% power conversion efficiency.

Determination of Effective Photon Lifetime in Nitrogenic Laser in One and Two Dimension

Since one of the most valuable measurable parameters in laser, called effective cavity lifetime , gives useful information about laser, this paper aims to study the description of it dependency, τ_ph^eff, on geometrical characteristics of N2-laser, electrodes length and amplifier gap separation. First based on the studies carried out on it , an oscillator-amplifier laser is used which operates under moderate current density conditions; Then in order to obtain a theoretical relation for effective cavity lifetime and to demonstrate the mentioned dependency using rate equations, at first a one-dimensional method is used for the photon density. Since the answers of rate equations in an oscillator-amplifier laser are complicated, a single-oscillator based modelis offered to make rate equations simpler. In this model, at first it is supposed that the photon density of inner part of the amplifier could benph (z,t)= nph (0,t) exp (g0(z)z), If nph≅ nph (z,t), then rate equations are used for this density and since g0 is a function of z or amplifier electrode length (Z≅lAMP), the cavity effective life time is calculated for equivalent oscillator.Then ,Since most of studies carried out in one dimension , so for approaching to more actual system a two -dimensional method is used for the photon density. So, we consider Z andY, which Z is along amplifier electrodes length and Y is along gaps separation. Supposing that Z and Y are independent on the photon density, two independent relations can be considered for the photon density. In this step, 2-dimensional photon density could be regarded as:nph (z,y,t) = nph (z,t) nph (y,t) . and then 2-dimensional effective cavity lifetime amount is obtained as:〖〖(τ〗_eff^ph)〗^(-1)=c/l_AMp (1+ ץ_l^z+bl_AMp ץ_l^z )+c/d_AMp (ץ_l^y+ad_AMp ץ_l^y ), This relation includes 2 independent values along the electrodes length (Z≅lAMP) and gap separation (y≅dAMP). It also demonstrates that the obtained 2-dimentional relation represents a perfect schema for lifetime behavior. The results of this calculation are consistent with other reported N2-laser effective cavity lifetime values measured under moderate current density conditions.

Investigations on correlation between I–V characteristic and internal quantum efficiency of blue (AlGaIn)N light-emitting diodes

We have studied the electrical and optical characteristics of (AlGaIn)N multiple quantum well light-emitting diodes. Minimizing contact effects by utilizing platinum as p-contact metal, ideality factors as low as 1.1 have been achieved. In agreement with basic semiconductor theory, a correlation between ideality factor and small-current efficiency was found. We were able to emulate the experimental current-voltage characteristic over seven orders of magnitude utilizing a two diode model. This model enables a very good prediction of internal quantum efficiency at moderate current densities out of purely electrically derived parameters.

Morphologically controlled electrodeposition of CdSe on mesoporous TiO2 film for quantum dot-sensitized solar cells

CdSe quantum dots (QDs)-sensitized mesoporous TiO2 (TiO2/CdSe) films were fabricated using a facile one-step electrodeposition method in an aqueous electrolyte. This technique has the advantage of being simple, low cost, and easily scalable to the sensitization of large-area panels. By adjusting the electrodeposition current density, the morphology and microstructure of the prepared TiO2/CdSe films can be precisely controlled, which influences the photovoltaic performances of quantum dot-sensitized solar cells based on the TiO2/CdSe films. At a moderate current density of 0.2mAcm−2, CdSe QDs can penetrate deep into the inner pores of the mesoporous TiO2 film, thus leading to a dense and uniform distribution of QDs throughout the whole TiO2 matrix, while higher current densities result in growth of larger, isolated CdSe nanoclusters. Furthermore, a ZnS passivation layer coated on TiO2/CdSe photoanodes and thermal annealing could significantly improve the photovoltaic performance. As a result, a quantum dot-sensitized solar cell based on a TiO2/CdSe/ZnS photoanode (350°C, 30min calcination), polysulfide electrolyte and Pt counter electrode achieves a power conversion efficiency of 2.72% under AM 1.5G one sun illumination.

The Structural and Electrochemical Impact of Li and Fe Site Substitution in LiFePO4

The crystal structure and delithiation mechanism of Li-site substituted LiFePO4 have been revealed by investigation of supervalent V3+ substitution. The combined X-ray and neutron powder diffraction data analysis surprisingly shows that the substituting aliovalent vanadium ions occupy the Fe site while some of the Fe resides at the Li site, probably as sarcopside, which leads to an increase in the unit cell volume. Such substitution reduces the miscibility gap at room temperature and also significantly lowers the solid solution formation temperature in the two-phase region. The effect of the phase diagram modification results in improved kinetics, leading to better rate performance. Such substitution, however, significantly lowers the LiFePO4 capacity at moderate current densities.

Recent insights concerning DCFC development: 1998–2012

We present an overview of recent developments of the Direct Carbon Fuel Cell (DCFC) cell and system technology which we believe are key to the worldwide renewal of interest in the DCFC during the last ten years. The importance of understanding and exploiting the co-production of CO and CO2 are examined. A distinction must be made between, on the one hand, the tendency toward chemical and electrochemical equilibrium and on the other hand the complex effects of chemical and electrochemical inhibition. The tendency toward equilibrium may be very active in the DCFC anode, resulting in high CO/CO2 ratios at high temperature and/or at low current density, consistent with the Boudouard equilibrium. The complex inhibitive effects tend to produce predominantly CO2 at moderate temperature and moderate current density. If the DCFC anode is allowed to come close to equilibrium, electrochemical production of CO may result. It is accompanied by a large increase in entropy compensated by absorption of thermal energy. This approach to equilibrium may be desirable, for example, in energy conversion systems where the absorption of thermal energy can be ensured via solar collectors. In that case, the product CO may be electrochemically converted or used for chemical or heating value. Such systems can reach an efficiency of greater than 80%. On the other hand, by inhibiting the Boudouard equilibrium either within the reaction mechanism or in the gas product in contact with carbon, it is possible to promote, even at relatively high temperature (700–750 °C), the 4-electron conversion of carbon to CO2, resulting in very high conversion efficiency (70–80%). Recent work has pinpointed the conditions under which a DCFC operating at high Coulombic efficiency can be realized. Such a power source of very high energy density, provided it also has sufficient power density, may compete with commercially available batteries and fuel cells in electrical storage and conversion applications. The molecular structure characteristics and kinetics yielding favorable conditions for this type of operation are discussed in detail, together with the optimal operating conditions for this mode of DCFC application.

Optimisation of electrocatalytic dechlorination of 2,4-dichlorophenoxyacetic acid on a roughened silver–palladium cathode

Electrocatalytic hydrogenolysis (ECH) dechlorination of 2,4-dichlorophenoxyacetic acid (2,4-D) in an aqueous solution was investigated at room temperature using a roughened silver–palladium cathode (Pd/Ag(r) cathode) in batch-mode electrolysis experiments. The Pd/Ag(r) cathode was prepared by galvanic replacement reaction (GRR) of a roughened silver (Ag(r)) electrode with PdCl2 solution. The effect of preparation conditions on the catalytic activity and stability of the Pd/Ag(r) cathode and of operating parameters on the rate and current efficiency (CE) of the ECH dechlorination reaction were evaluated. In particular, the ECH dechlorination mechanism of 2,4-D was analysed with regard to the dependence of dechlorination efficiency on the different operating parameters. Moreover, preliminary assessments of product selectivity and carbon mass balance of the dechlorination reaction were carried out. The results demonstrate that a moderate GRR time and GRR temperature favoured the catalytic activity and cathode stability and that a basic aqueous solution without ethanol, high 2,4-D concentration, and moderate current density had the most beneficial effects on the dechlorination process. Under the optimised conditions, 25mM of 2,4-D could be selectively dechlorinated to phenoxyacetic acid with 85% yield and 66% CE at 298K after 6h electrolysis. The only products generated during the electrolysis process were phenoxyacetic acid, 2-chlorophenoxyacetic acid, and 4-chlorophenoxyacetic acid.

Extremely stable all solution processed organic tandem solar cells with TiO2/GO recombination layer under continuous light illumination

One approach to harvest a wide solar spectral solar energy is to stack two solar cells with different absorption characteristics in a tandem cell architecture. Herein, solution processed tandem solar cells, with highly transparent titanium oxide (TiO2) and graphene oxide (GO) as an efficient recombination layer, were designed, fabricated and characterized. We have adopted poly[(4,4'-bis(3-ethylhexyl)dithieno[3,2-b:''3'-d]silole)-2,6-diyl-alt-(2,5-(3-(2-ethylhexyl)thiophen-2-yl)thiazolo[5,4-d]thiazole]:indene-C60 bisadduct (PSEHTT:ICBA) and poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl]:[6,6]-phenyl-C70 butyric acid methyl ester (PSBTBT:PC70BM) as the active layers for the front and rear cells, respectively. The TiO2/GO serves as an electron and hole collecting and recombination layer. The tandem solar cells showed a high open circuit voltage (VOC) 1.62 V, a moderate short circuit current density (JSC) 8.23 mA cm(-2), fill factor (FF) 62.98%, leading to the power conversion efficiency of 8.40%. The obtained VOC value of tandem solar cells is ideal for the summation of VOCs attained from front and rear cells and it is evident that our tandem solar cells are well connected in series. Moreover, this tandem cell exhibits a 20% drop in conversion efficiency under continuous AM illumination for 2880 h.

鉄置換Li<sub>2</sub>MnO<sub>3 </sub>系正極材料の合成と充放電特性に及ぼすチタン置換効果

Fe- or Fe and Ti-substituted Li2MnO3 positive electrode material (FM or FMT sample) with monoclinic Li2MnO3 type structure (C2/m) was synthesized using coprecipitation-hydrothermal-calcination. Both materials exhibited high specific capacity greater than 200 mAh/g against a Li metal negative electrode at moderate current density of 40 mA/g between 2.0 and 4.8 V using stepwise charging. The cycle performance for the FMT sample (Li1+x(Fe0.2Ti0.2Mn0.6)1−xO2, 0 < x< 1/3) using a graphite negative electrode was better than that for the FM sample (Li1+x(Fe0.2Mn0.8)1−xO2, 0<x<1/3), indicating that partial substitution of Ti4+ for Mn4+ ion is effective for improving cycle performance under the practical lithium-ion cell configuration. The results presented above demonstrate that an Fe-substituted Li2MnO3 based positive electrode material can become an attractive candidate as an inexpensive constituent material for a large-scale lithium-ion battery by selecting suitable additional elements such as Ti and electrochemical charge conditions for activation.